This sudden or very rapid expansion of volume is attended by an exhibition of force which is more or less violent, according to the constitution of the original body and the circumstances of the explosion.

Any substance capable of undergoing such a change on the application of heat or other disturbing cause is called an "explosive."

Explosive Force.—Explosive force is directly proportional to the heat of combustion and the volume of gas, and inversely to the specific heat of the mixed products.

Explosive effect is directly proportional to the volume of gas produced and the temperature of the explosion, and inversely as the time required for the change to take place.

Explosive Effect and Force compared.—Explosive effect depends upon the rapidity with which the conversion is effected, while the same amount of explosive force may act suddenly or gradually.

As before stated, explosions are more or less violent according to the circumstances under which they take place. These may be considered as follows:—

The Physical State of the Explosive Substance.—Numerous instances may be cited to show the influence the physical condition of an explosive body has upon its explosion.

Thus, gunpowder may, by merely varying the size, shape, and density of the grain, be made to ignite rapidly but burn comparatively slowly, or be made to ignite more slowly, but once inflamed to burn very rapidly.

Again, gun-cotton in a loose, uncompressed state, will, if ignited, only flash off; if it is spun into threads or woven into webs, its rate of combustion may be so much reduced that it can be used in gunnery or for a quick fuze; while if powerfully compressed and damp it burns slowly. Wet gun-cotton requires a primer of dry gun-cotton and a fulminate fuze to explode; dry, it may be exploded by a fulminate fuze, &c.

Then nitro-glycerine, when exploded by 15 grains of fulminate of mercury, and at a temperature above 40° F., is very violently detonated; below 40° F. it freezes and cannot be similarly exploded.

To obtain the full effect of all explosives, confinement is absolutely necessary.

The more rapid the explosion the less confinement required, approaching in the case of some explosives to so small an amount that it need not, for practical purposes, be considered.

Thus a charge of nitro-glycerine or gun-cotton, when detonated in the open air, will destroy wrought iron rails, large blocks of stones, balks of timber, &c.

In the case of the former body, the confinement of the atmosphere is sufficient.

In the latter, the mechanical cohesion due to compression is sufficient restraint.

Abel states that if the film of atmosphere surrounding the nitro-glycerine, not exceeding 1/1000 inch in thickness, be removed, the explosive effect is much lessened.

A large charge of gunpowder fired in the ordinary way under water requires a strong case to retain the gases until the action has become general, or, owing to its slow rate of burning, the case would be broken before the whole of the charge had been ignited, and part of the charge drowned.

This is often to be noticed when firing fine-grained powder in heavy guns.

Igniting the charge at several points diminishes the confinement needed.

Mode of Firing.—The application of heat, directly or indirectly, is the principal means of causing an explosion.

The flame from a percussion cap or primer, or a platinum wire heated to incandescence by an electric current, will directly ignite a charge. Friction, concussion, &c., will indirectly ignite a charge due to the conversion of mechanical energy into heat.

It would appear that when one explosive body is used as a means of firing another, the resultant explosion is due to the blow suddenly formed by the gas of the firing charge acting percussively upon the mass to be exploded. If such were the case, then the most powerful explosive would be the best agent for causing an explosion. But it is not so.

For example, nitro-glycerine, which is far more powerful than fulminate of mercury, requires more than 1000 grains to explode gun-cotton, while only 15 grains of the latter is needful for the same work, &c.

A small quantity of an explosive substance which is sensitive to friction or percussion is often used to ignite the original charge.

Detonation.—The instantaneous explosion of the whole mass of a body is defined as "detonation."

The essential difference between an explosion and a detonation is the comparative suddenness of the transformation of the solid or liquid explosive substance into gas and vapour.

Some explosive bodies, such as the fulminates, &c., always detonate, while the detonation of others depends on the mode of firing.

Nitro-glycerine always explodes violently, but when fired with an initiatory charge of fulminate of mercury it is much more powerful than when fired with gunpowder.

Compressed gun-cotton in the air-dry state can be detonated by 2 grains of fulminate of mercury embedded in the material, but when it contains 3 per cent. of water over and above the 2 per cent. which exists normally in the air-dry substance, 15 grains of the fulminate will not always do so.

Theory of Detonation.—The theory of detonation is not yet thoroughly understood. That it is not alone due to the heat caused by the impact of the mechanical energy of the particles of gas, set free from the initiatory charge on the principal mass, is proved by the fact of its being possible to detonate wet gun-cotton.

Professor Bloxam terms detonation to be "sympathetic" explosion.

Experiments carried on in England by Professor Abel, and in France by MM. Champion and Pellet, tend to show that it is due to the vibratory action of the detonating agent.

Thus a glass may withstand a strong blow, though a particular note or vibration will smash it.

All explosive compounds and mixtures, including gunpowder, are susceptible of violent explosion through the agency of a detonation.

Roux and Sarrau.—Roux and Sarrau divide explosions into two orders:—

Simple explosions are produced by direct inflammation, or by a small charge of gunpowder.

Detonations are obtained from nitro-glycerine, gun-cotton, &c., by exploding with fulminate of mercury.

They state that fulminate of mercury does not detonate gunpowder; but if the exploding charge is a small amount of nitro-glycerine, itself detonated by fulminate of mercury, then an explosion of the first order is obtained.

The relative effects were approximately measured by determining the quantities necessary to rupture small cast iron shells of supposed equal strength.

Results of their Experiments.—The following are some of the results:—

According to the above table, nitro-glycerine is more than ten times, and gun-cotton more than six times, as powerful as gunpowder fired in the ordinary way (2nd order).

The want of reciprocity between two detonating agents is shown in a remarkable degree by the following experiments, carried out by Professor Abel:—

Explosive agents are divided into explosive mixtures and compounds.

In the former the ingredients are mechanically mixed, and can be separated by mechanical means.

In the latter the ingredients are chemically combined, and can only be separated by chemical change.

Torpedo Explosive Agents.—The explosive agents that are practically the most important, as far as their employment as torpedo charges are concerned, are as follows:—

Explosive Mixtures.—A.—Explosive mixtures.

Explosive Compounds.—B.—Explosive compounds.

• 1.—Nitro-glycerine.
• 2.—Dynamite (No. 1).
• 3.—Gun-cotton.
• 4.—Fulminate of mercury.

A.—Explosive Mixtures.

Gunpowder.—This explosive mixture is composed of seventy-five parts of nitre (saltpetre), fifteen parts of charcoal, and ten parts of sulphur.

On being ignited, the oxygen which is feebly held by the nitrogen combines with the carbon, forming carbonic oxide gas, whilst the sulphur unites with the potassium of the nitre, the whole combination being accompanied by a great evolution of heat and expansion of gas, and the nitrogen is set free.

Properties, &c.—A spark, friction between hard bodies, or a temperature of 572° F., are any of them sufficient to cause an explosion of gunpowder.

Slight moisture, due to damp air, &c., produces caking and deterioration.

Wetting causes permanent destruction.

Frost does not injure it.

It can be fired by ordinary methods.

It can be transported and handled with safety and great ease.

It is not a suitable explosive agent for torpedoes, on account of its liability to be injured by damp, as well as its not being sufficiently violent, though for the sake of convenience, &c., it is often employed for such work.

The effect produced by the explosion of a charge of gunpowder, ignited by the ordinary method, is that of an uplifting rather than a shattering effect.

This evil may be greatly remedied, when gunpowder is used as the charge of a torpedo, by firing it with a detonator, by which means its fullest explosive effect is developed.

Picric Powder.—The picrates are salts of picric acid.

Picric acid is formed by the action of nitric acid on carbolic acid.

The picrate employed by Professor Abel is prepared from picric acid and ammonium. This preparation, or salt mixed with nitre (saltpetre), forms Abel's picric powder.

Properties, &c.—It is prepared for use in a similar manner to gunpowder, and it can be handled in the same way.

It is less violent than dynamite or gun-cotton, though much more so than gunpowder.

It is difficult to explode it by blows or friction.

If flame be applied to it, the part touched burns, but the combustion does not become general.

This explosive agent will probably be used for spar torpedoes, when gun-cotton or dynamite are not employed.

B.—Explosive Compounds.

Nitro-glycerine.—Nitro-glycerine is formed by the action of nitric acid upon glycerine at a low temperature.

The manufacture of this compound consists, first, in the slow mixture of the glycerine with the acid, at a low temperature; secondly, in washing the nitro-glycerine from the excess of acid with water.

The nitric acid before use is mixed with a certain proportion of strong sulphuric acid, so that the water formed during the reaction may be taken up, and thus any dilution of the nitric acid is prevented.

Nitro-glycerine is composed of carbon, hydrogen, nitrogen, and oxygen, as indicated by the equation C₃H₅N₃O₉.

Properties, &c.—At ordinary temperatures nitro-glycerine is an oily liquid, having a specific gravity of 1·6. Freshly made it is creamy white and opaque, but clears and becomes colourless on standing for a certain time, depending on the temperature.

It does not mix with, nor is it affected by, water. It has a sweet, aromatic taste, and produces a violent headache if placed upon the tongue.

The opaque, freshly made nitro-glycerine does not freeze until the temperature is lowered to 3°-5° below zero, F., but, when cleared, it freezes at 39°-40° F. Nitro-glycerine freezes to a white crystalline mass, and in this state it can be thawed by placing the vessel containing it in water, at a temperature not over 100° F.

If flame is applied to freely exposed nitro-glycerine, it burns slowly without explosion.

Nitro-glycerine in a state of decomposition becomes very sensitive, exploding violently when struck, even when unconfined.

Pure nitro-glycerine does not spontaneously decompose at any ordinary temperature, but if it contains any free acid, then decomposition may happen. When pure, it is not sensitive to friction, or moderate percussion. If struck with a hammer, only the particle receiving the blow explodes, the remainder being scattered.

The firing point of nitro-glycerine is about 356° F., though it begins to decompose at a lower temperature.

The mode of firing nitro-glycerine usually employed is that of a fulminate of mercury detonating fuse.

Nitro-glycerine in the frozen state cannot be fired even by large charges of fulminate.

In one instance, 1600 lbs. of liquid nitro-glycerine exploded in a magazine containing 600 lbs. of the same substance in a frozen state, but failed to fire the latter, only breaking it up and scattering it in every direction.

Dynamite.—This explosive compound is merely a preparation in which nitro-glycerine is itself presented for use, its explosive properties being those of the nitro-glycerine contained in it, as the absorbent is an inert body.

Dynamite is formed of seventy-five parts of nitro-glycerine absorbed by twenty-five parts of a porous siliceous earth or "kieselguhr."

The best substitute for "kieselguhr" is ashes of bog-head coal.

Dynamite is a loose, soft, readily moulded substance, of a buff colour.

The preparation of dynamite is very simple.

The nitro-glycerine is mixed by means of wooden spatulas with the fine white powder (kieselguhr) in a leaden vessel.

It freezes at 39°-40° F., and when solidly frozen cannot be exploded, but if in a pulverised state it can be exploded, though with diminished violence.

It can be easily thawed, by placing the vessel containing it in hot water.

Friction or moderate percussion does not explode it.

Its firing point is 356° F.

If flame be applied to it, it burns with a strong flame.

It is fired by means of fulminate of mercury, and its explosive force is about seven times that of gunpowder.

For ground and buoyant mines, where actual contact between the hostile vessel and the torpedo will be rarely achieved, this being next to nitro-glycerine the most violent of all known explosive agents, and being cheaply and readily procured, is the very best explosive for such torpedoes.

That it is not generally adopted is owing to its containing a large proportion of that seemingly dangerous substance, nitro-glycerine, which makes the handling of dynamite a somewhat hazardous operation.

According to Professor Abel, there are now as many as fifteen dynamite factories in different parts of the world (including a very extensive one in Scotland) working under the supervision of Mr. Nobel, the originator of the nitro-glycerine industry; and six or seven other establishments exist where dynamite or preparations of very similar character are also manufactured.

The total production of dynamite in 1867 was only eleven tons, while in 1878 it amounted to 6140 tons.

This explosive compound is most extensively used for general blasting purposes all over the world, and for this purpose, owing to its cheapness and the convenience in manipulating it, is far superior to compressed gun-cotton.

Gun-cotton is formed by the action of concentrated nitric acid on cotton, its composition being indicated by the formula CH₇(NO₂)₃O₅.

Professor Abel's process for manufacturing pulped and compressed gun-cotton is as follows:—

Cotton waste is the form of cotton used; it is picked and cleaned, thoroughly dried at 160° F., and then allowed to cool.

The strongest nitric and sulphuric acids are employed, mixed in the proportion of one part of the former to three of the latter by weight. These are mixed in large quantities, and stored in cast-iron tanks.

The cotton in 1-lb. charges is immersed in the acid mixture, which is contained in a trough surrounded by cold water. After being subjected to the action of the acid for a short space of time, the cotton is taken up, placed upon a perforated shelf, and as much as possible of the acid squeezed out of it. It is then put into jars, covered with fresh acid, and the jars placed in fresh water, remaining there for twenty-four hours.

To remove the acid, the gun-cotton from the jars is thrown into a centrifugal strainer, by which nearly all the acid is expelled. It is then diffused quickly in small quantities through a large volume of water, and again passed through a centrifugal machine.

The next process is that of thoroughly washing the gun-cotton, for the purpose of removing the traces of the acid still adhering to it. By pulping, which operation is performed in pulping engines or beaters, the washing is expeditious and thorough.

A beater is an oblong tub in which is placed a revolving wheel carrying strips of steel on its circumference. From the bottom under the wheel project similar steel strips.

The action of this machine is as follows:—

By the rotation of the wheel, the gun-cotton which is suspended in water circulates around the tub, and is drawn between the two sets of steel projections, by which it is reduced to a state of pulp.

The bottom of the tub is movable, and thus the space through which the gun-cotton must pass may be contracted, as the operation proceeds.

The pulping being complete, the contents are run into poachers for the final washing.

A poacher is a large oblong wooden tub. On one side at the middle is placed a wooden paddle-wheel, which extends half way across the tub.

In the poacher the pulped gun-cotton is stirred for a long time with a large quantity of water. The revolution of the paddle-wheel keeps up a constant circulation, and care is taken that no deposit occurs in any part of the tub.

Having converted the cotton into gun-cotton, reduced it to a state of pulp, and thoroughly washed it, the next process is to separate the water from the pulp, and compress it into cakes or discs.

This is accomplished by means of two presses, the first of which has 36 hollow cylinders, in which perforated plungers work upwards.

These plungers having been drawn down, the cylinders are filled with the water-laden pulp, and their tops covered with a weight; the plungers are then forced up by hydraulic power, compressing the pulp, and forcing the water to escape through their perforations.

The second one is used to more solidly compress the cylindrical masses of gun-cotton formed by the action of the first press, a pressure of 6 tons to the inch being in this case applied.

About 6 per cent. of moisture still remains in the discs, which can be readily removed by drying.

Properties.—Cotton converted into gun-cotton is little changed in appearance, though the latter is harsher to the touch than the former.

If a flame be applied to dry loose gun-cotton, it flashes up, without explosion; if compressed it burns rapidly, but quietly.

Moist compressed gun-cotton under the same circumstances burns away slowly.

Gun-cotton containing 12 to 14 per cent. of water is ignited with much difficulty on applying a highly heated body. As it leaves the hydraulic press upon being converted from the pulped state to masses, it contains about 15 per cent. of water; in this condition it may be thrown on to a fire or held in a flame without exhibiting any tendency to burn; the masses may be perforated by means of a red-hot iron, or with a drilling tool, and they may with perfect safety be cut into slices by means of saws revolving with great rapidity. If placed upon a fire and allowed to remain there, a feeble and transparent flame flickers over the surface of the wet gun-cotton from time to time as the exterior becomes sufficiently dry to inflame; in this way a piece of compressed gun-cotton will burn away very gradually indeed.

To test the safety of wet gun-cotton, the following two experiments among many have been made:—

Quantities of wet gun-cotton, 20 cwt. each, packed in one instance in a large, strong wooden case, and in the other in a number of strong packing cases, were placed in small magazines, very substantially built of concrete and brickwork. Large fires were kindled around the packages in each building, the doors being just left ajar. The entire contents of both buildings had burned away, without anything approaching explosive action, in less than two hours.

This comparatively great safety of wet gun-cotton, coupled with the fact that its detonation in that state may be readily accomplished through the agency of a small quantity of dry gun-cotton, termed a "primer," which, by means of a fulminating fuze, or detonator, is made to act as the initiative detonating agent, gives it important advantages over other violent explosive agents, when used for purposes which involve the employment of a considerable quantity of the material, on account of the safety attending its storage and necessary manipulation.

From experiments conducted by engineer officers in Austria, it was found that if boxes containing dry compressed gun-cotton are fired into from small arms, even at a short range, the gun-cotton is generally inflamed, but never exploded, the sharpness of the blow essential to effect an explosion, which the bullet might otherwise give, being diminished by its penetration through the side of the box before reaching the explosive. Wet gun-cotton, containing even as little as 15 per cent. of water, is never inflamed on these conditions.

Dynamite, on the other hand, is invariably detonated when struck by a bullet on passing through the side of the box.

Gun-cotton is insoluble in and unaffected by water.

The firing point of gun-cotton is about 360° F.

The temperature of explosion of gun-cotton is about 8700° F., being more than double that of gunpowder. Gun-cotton is not sensitive to friction or percussion.

If not perfectly converted or thoroughly washed, gun-cotton is liable to spontaneous decomposition, which under favourable conditions may result in explosion.

Compressed gun-cotton is free from such danger, as it may be kept and used saturated with water. It is stored in the wet state, care being taken that it is not exposed to a temperature that will freeze the water in the cakes, as if this occurs they are liable to be disintegrated by the expansion of the water in freezing.

Gun-cotton is the agent most extensively used for all kinds of military engineering and submarine operations in Great Britain, it being especially manufactured by the English government for that express purpose; but in other countries it is not so manufactured, and therefore, as it is little used for other than military purposes, it is not to any extent privately manufactured, as is the case with other explosives, such as dynamite, dualine, lithofracteur, &c., and thus, in case of war, would be somewhat difficult to obtain out of England.

Compared with dynamite, it is not so violent, and occupies more space, weight for weight, and also requires a more complicated means of detonating it. On the other hand, gun-cotton is infinitely safer to store and manipulate, and is not so subject to detonation by concussion (not being so sensitive) as dynamite.

Fulminate of Mercury.—Fulminate of mercury is formed by the action of mercuric nitrate and nitric acid upon alcohol. The mode of preparation is as follows:—

Dissolve one part of mercury in twelve parts of nitric acid, and pour this solution into twelve parts of alcohol.

Pour this mixture into a vessel which is placed in hot water until it darkens and becomes turbid and begins to evolve dense white fumes, then remove it from the water. The reaction goes on, with strong effervescence and copious evolution of dense white ethereal vapours. If red fumes appear, cold alcohol should be added to check the violence of the action.

The operation should be performed at a distance from a fire or flame, and in a strong draught, so that the vapours may be carried off.

When the liquid clears, and the dense white fumes are no longer given off, further action is stopped by filling up with cold water. The fulminate settles to the bottom of the vessel as a grey crystalline precipitate. The liquid is then poured off, and the fulminate washed several times by decantation or upon a filter.

Dry fulminate of mercury explodes violently when heated to 367° F., when forcibly struck by the electric spark, &c.

When wet it is inexplosive, and therefore it is always kept wet, being dried in small amounts when required for use.

Fulminate of mercury is applied in many ways, either pure or mixed with other substances, as in percussion caps, percussion powder, primers, detonators, &c.

For the purpose of detonating nitro-glycerine or its preparations, 15 grains of the fulminate are sufficient, but to detonate gun-cotton 25 grains are necessary. The fulminate in detonating fuzes should be enclosed in a copper case or cap, and must never be loose. The fulminate should be wet when charging the detonators, as it is very dangerous to handle when dry.

Great care is requisite in handling this explosive compound.

In addition to the foregoing explosive compounds and mixtures, the following explosive agents have also been employed for the purposes of submarine operations, though only to a small extent.

Dualin.—Dualin is a nitro-glycerine preparation formed by mixing sawdust and saltpetre with that substance.

This preparation, inferior to dynamite, was employed by the Germans as the explosive agent for their submarine mines during the Franco-German war (1870-71).

Lithofracteur.—Lithofracteur is also a preparation of nitro-glycerine. It is composed of the following materials:—Nitro-glycerine, kieselguhr, coal, soda, saltpetre, and sulphur.

This explosive agent, also inferior to dynamite, is used, though not very extensively, by the French for their submarine mines.

Horsley's Powder.—Horsley's powder is a chlorate mixture formed of potassium, chlorate, and galls. This explosive mixture was formerly used by Captain Harvey for his towing torpedo, but has recently been discarded for compressed gun-cotton.

Abel's Detonation Experiments.—The following are the results of experiments carried out by Professor Abel, C.B., F.R.S., on the subject of detonation:—

Explosive Agents in Torpedoes.—The explosive agents that at the present time are most generally used in torpedoes are gunpowder, gun-cotton in the wet compressed state, and dynamite, and these may be compared as to their properties and their explosive effects.

Gunpowder.—Gunpowder is a familiar material, in general use for all military purposes. It can be handled and transported with safety and ease, and it can be fired by ordinary methods. But for submarine purposes it has the disadvantage of being very easily injured by water, so that it is absolutely necessary to enclose it in water-tight cases.

Gun-cotton.—Gun-cotton is free from liability to accidents, and in this matter, and the safety of its manufacture, it compares favourably with gunpowder.

It is peculiarly adapted to submarine work, being unaffected by water. And as it may be kept in water, ready for use, it can be safely carried on board ship in large quantities. It is far more violent in its action when detonated than gunpowder. The chief objection to its use is, that being applied only for special purposes, it is not readily obtained. Also it requires a peculiar and somewhat complicated mode of firing it.

Dynamite.—Dynamite is more easily manufactured than the two foregoing explosives. The fact of it containing nitro-glycerine, which has a bad reputation, has militated against its use as a torpedo explosive agent, though for blasting purposes it is most extensively used. Though not directly affected by water, its firing is hindered when diffused through water. Another disadvantage is its high freezing point. Like gun-cotton, it requires special means to fire it, though much simpler, and also is much more powerful than gunpowder. The explosive effect of dynamite or gun-cotton is a rending or a shattering one, while that of gunpowder is an uplifting or heaving one.

Again, it is necessary when using gunpowder that the object be in the line of least resistance, but with dynamite or gun-cotton the effect is nearly equal in every direction, therefore for submarine operations, either dynamite or gun-cotton is the explosive agent that should be invariably used.

Size of Torpedo Charges.—For permanent mines, a charge of 700 lbs. to 1000 lbs. of gun-cotton is quite sufficient, though too large a charge cannot be employed, except as regards the matter of convenience.

For buoyant mines, 500 lbs. to 700 lbs. of gun-cotton is an ample charge, and for contact mines, 200 lbs. to 300 lbs. of gun-cotton is sufficient. In spar torpedoes, where lightness is a consideration, gun-cotton charges of 30 lbs. to 50 lbs. will be found ample, and similarly in the case of the towing or locomotive torpedoes. Of course, with regard to such a submarine weapon as the Lay torpedo boat, any size charge may be carried, according to the wish of the builder.

Torpedo Explosions illustrated.—At Fig. 166 is represented a sketch of a torpedo explosion, from a photograph taken at the moment the column of water was at its greatest elevation. The torpedo contained 432 lbs. of gun-cotton, and was exploded under 27 feet of water.

The height of the column thrown up measured 81 feet, and the diameter at the base 132 feet.

At Fig. 165 is shown a sketch of two submarine mine explosions from an instantaneous photograph; the schooner which is shown in the sketch happened to be passing at the moment of explosion, thus affording a comparison as to the size of the columns of water thrown up.

The column on the left was due to the explosion of a submarine mine containing 100 lbs. gunpowder at a depth of 10 feet below the surface. That on the right was the result of an explosion of a similar mine, but at a depth of 41 feet below the surface. Its extreme height was 400 feet.